A resource in an LTE (Long Term Evolution) or LTE-A (LTE Advanced) system is classified into a radio frame (a system frame) in terms of time, where one radio frame includes 10 subframes, a length of one subframe is 1 ms, and one subframe includes two timeslots; and is classified into a subcarrier in terms of frequency. A physical resource block (PRB) is defined as a timeslot in terms of time and 12 subcarriers in terms of frequency.
In a radio communications system, user equipment (UE)/terminal needs to establish a connection to a network, and this process is generally referred to as a random access process. In the LTE or LTE-A system, the random access process is classified into two types: a contention-based random access process and a non-contention based random access process. The contention-based random access process includes four steps: Step 1. The UE sends a random access preamble (a preamble for short) to a base station. Step 2. The base station returns a random access response to the terminal. Step 3. Message 3 (Msg 3). Step 4. Contention resolution message (Msg 4). The non-contention based random access process includes only the first two steps.
In an LTE or LTE-A network with coverage enhancement, enhanced transmission needs to be performed on a physical random access channel (PRACH), and PRACH enhanced transmission refers to resending, in a random access process, a random access preamble according to an existing preamble format. When existing LTE/LTE-A network coverage needs coverage enhancement of additional x dB (for example, x=15 or 20), actually, because distances between different UEs and the base station are different, channel environments between the different UEs and the base station are different, and path losses between the UEs and the base station are also different, a required system coverage enhancement value may be a value in 0-x dB for the different UEs. To implement connection establishment between the different UEs and the network, quantities of repetition times required by the UEs to send a preamble are also different.
Currently, a solution to implementing PRACH enhanced transmission is as follows: there are multiple repetition levels of PRACH enhanced transmission for preamble resending, for example, repetition levels 1, 2, and 3 of PRACH enhanced transmission. A quantity of repetition times for sending a preamble by UE at each repetition level is specified in advance by the system or signaled. At different repetition levels of PRACH enhanced transmission, quantities of repetition times for sending a preamble by the UE are different. When a required system coverage enhancement value is larger, a path loss between the UE and the base station is also larger, a repetition level that is of PRACH enhanced transmission and that is required to be used by the UE to successfully complete a random access process is also higher, and the quantity of repetition times for sending the preamble is larger. At each repetition level, the UE resends the random access preamble according to a quantity of repetition times for the repetition level.
For each repetition level of PRACH enhanced transmission, a resource set of PRACH enhanced transmission is specified in advance by the system or signaled. The resource set of PRACH enhanced transmission includes one or more resources of PRACH enhanced transmission. The resources of PRACH enhanced transmission include a code resource (a preamble), a time resource, and a frequency resource that are used to send the preamble. The time resource and the frequency resource may be collectively referred to as a time-frequency resource. During PRACH enhanced transmission, the UE sends the preamble by using one repetition level of PRACH enhanced transmission, and resends, according to a quantity of repletion times for the repetition level and according to an existing preamble format, the preamble on a resource that is of PRACH enhanced transmission and that is included in a resource set for the repetition level.
For PRACH enhanced transmission, a time resource and/or a frequency resource included in a resource set of PRACH enhanced transmission may be the same as a time resource and/or a frequency resource for PRACH transmission (PRACH transmission that does not need to be enhanced, and a preamble is sent according to an existing format without repetition). In PRACH enhanced transmission and PRACH transmission, resource multiplexing is performed in a code division multiplexing (CDM) manner, that is, the resource set of PRACH enhanced transmission and a resource set of PRACH transmission include different code resources (preambles). The time resource and/or the frequency resource included in the resource set of PRACH enhanced transmission may also be newly defined, and are different from the time resource and/or the frequency resource of PRACH transmission.
At each repetition level of PRACH enhanced transmission, resending a preamble by UE according to a quantity of repetition times for the repetition level is denoted as one preamble sending attempt. If the UE does not receive an RAR after completing one preamble sending attempt at a repetition level of PRACH enhanced transmission (not a highest repetition level of PRACH enhanced transmission), the UE increases the repetition level of PRACH enhanced transmission by one level, and performs a preamble resending attempt according to a quantity of repetition times for an increased repetition level of PRACH enhanced transmission.
Transmit power PPRACH for sending the random access preamble by the UE is obtained according to the following formula: PPRACH=min{PCMAX,c(i), PREAMBLE_RECEIVED_TARGET_POWER+PLc}_[dBm], where min{ } indicates an operation for calculating a minimum value, PCMAX,c(i) is a maximum value of transmit power of the UE in a subframe whose number is i, PLc is a value of a downlink path loss that is estimated by the UE, dBm is a power unit decibels per milliwatt, and PREAMBLE_RECEIVED_TARGET_POWER is preamble received target power and is obtained by calculating preambleInitialReceivedTargetPower+DELTA_PREAMBLE+(PREAMBLE_TRANSMISSION_COUNTER−1)*powerRampingStep, where preambleInitialReceivedTargetPower is preamble initial received target power, DELTA_PREAMBLE is a power offset value related to a preamble format, powerRampingStep is a power ramping step, and PREAMBLE_TRANSMISSION_COUNTER is a quantity of preamble transmission times.
In the foregoing PRACH enhanced transmission method, if transmit power is determined in the power calculation manner for each preamble sending attempt performed by the UE, there is a problem of a power waste of UE.